1. Field
This disclosure generally relates to structures having acoustically treated areas for noise reduction, and deals more particularly with a repair method that restores the acoustic performance of acoustically treated aircraft structures.
2. Background
High bypass type aircraft engines produce noise due to the high airflows through inlets, rotating stages and exhaust nozzles of the engines. In order to reduce noise and satisfy noise regulations governing commercial aircraft, high bypass engines may incorporate acoustic panels in various parts of the engine, such as in the inlets of engine nacelles. These acoustic panels, sometimes referred to as acoustic treatments, may comprise a septumized honeycomb core sandwiched between a perforated inner skin and a non-perforated outer skin. The inner and outer skins may comprise a metal such as aluminum, or a composite, and the honeycomb core may comprise a metal, a ceramic or a composite.
Acoustic panels of the type described above sometimes require localized repair due to impact damage. For example, the acoustically smooth inlets of engine nacelles may suffer impact damage due to bird strikes or other foreign objects and debris (FOD). One existing technique for repairing such impact damage involves splicing the honeycomb core using foaming adhesives, however this repair method is undesirable because it results in blockage of honeycomb cells and a loss of acoustic smoothness. Some types of aircraft have allowable damage limits of acoustically treated areas, but regulations may not permit structural repair of these areas. In other aircraft, non-structural repairs are permitted for limited areas of a panel that that restore its acoustic properties, however existing structural repair techniques that do not restore acoustic properties are only permitted on a time limited basis. In these latter mentioned aircraft, the entire panel must be replaced in the event that it sustains any degree of structural impact damage.
Accordingly, there is a need for a method of repairing acoustically treated areas of a structure such as inlets of an engine nacelle, which restores acoustic performance, and which may be efficiently carried out by service personnel with minimum downtime of aircraft.